1. Field of the Invention
This invention relates to electrolytic cells for the production of halogen from an aqueous alkali metal halide solution wherein the cell is of the diaphragm type and, more particularly, the invention relates to an improved diaphragm for that cell.
2. Description of the Prior Art
For many years, as shown by the textbook, Chlorine, ACS Monograph 154, J. S. Sconce, Editor, Reinhold Publishing Company, New York, New York, 1962, a diaphragm-type cell has been commonly used in the production of halogens such as chlorine and caustic from various alkali metal halide solutions. Among the cells disclosed therein are the well-known Hooker type cells, the Diamond type cells and filter press cells. Particularly for the Hooker and Diamond type cells it has been customary to use an asbestos deposited material as the diaphragm inasmuch as these diaphragms are fairly easily and consistently made. However, the use of asbestos has not been entirely satisfactory, especially with the advent of the newer, dimensionally stable metal anodes, inasmuch as the asbestos diaphragm tends to have a rather short service life; and now asbestos has become a suspect health hazard.
U.S. Pat. No. 3,694,281 discloses impregnating the asbestos diaphragm with polytetrafluoroethylene so as to obtain improved mechanical strengths when the diaphragm is employed under conditions of continuous operation in electrolytic cells containing an aqueous electrolyte. Another U.S. Pat. No. 3,312,614 discloses the use of a diaphragm made of polytetrafluoroethylene fabric for use in a filter press cell. The use of the fabric diaphragm is said to substantially preclude disturbances of the electrolysis plant because of overheating in the electrolytic cell. Still another U.S. Pat. No. 3,438,879 discloses a permselective diaphragm for an electrolytic cell wherein the diaphragm can be fabricated from polytetrafluoroethylene made as a woven cloth.
The textbook, Industrial Electrochemical Processes, edited by A. P. Kuhn, Elsevier Publishing Company, Amsterdam, 1971, in Chapter 15 at page 583 notes that a continuous mat of polytetrafluoroethylene fibers fused together at each intersection has been suggested for use in electrolytic cells; but its use as a filtering diaphragm would only become practical if the hydrophobic character which would cause occlusion of gas bubbles within the pores with a resultant increase in electrical resistance could be overcome.